Flush out AX15, or just change fluid?

Messages
1,565
Location
Elkridge, MD
Long story short, I filled the AX15 in my Jeep Cherokee with Mobil 1 75W-90 GL-5 gear oil about a year ago. After a few months, I noticed it was shifting a bit rougher. It was never great, and based on what I saw come out of the drain hole, I don't think the original owner ever changed the transmission fluid in its first 155K miles. Didn't think anything of it until I came across some comments on this here inter-web saying never to use a GL-5 fluid in the '96-'99 AX15's, because it eats the brass synchros. So, about a month ago, I drained the M1, and filled it with some Coastal GL-3,4,5 80W-90 fluid. Not surprisingly, its shifting even worse with this fluid. Then I read that GL-3,4,5 fluid really is just GL-5 fluid, and that, in fact, this stuff is even worse than the Mobil fluid I was using. Ugh. Today, I bit the bullet and ordered 4 quarts of Redline MT-90, for almost $50 after shipping. It best shift like butter when I put this stuff in there. The question is, should I drain the Coastal and maybe run some 10W-40 engine oil in there for a little while to clean out whatever sulfur was in the old stuff before I fill it with the MT-90? I'd like to get at least 20K out of this $50 worth of fluid, so I'm thinking maybe a "flush" with some nice, sulfur-free, engine oil isn't a bad idea.
 
Last edited:
Messages
4,434
Location
Massachusetts
I called Mobil a few years ago and I can confirm that they don't recommend their 80W90 for transmission use. I used to use Supertech 80W90, but the SM420 transmission in my Jeep never felt "right". Ironically, I tried Coastal 80W90 GL3,4,5 and it feels great. I also noticed that it doesn't smell as strong as the ST. I am currently searching for something new though, to hold up to extended 1st gear use (7.05:1 ratio). Someone recommended Mobil Delvac synthetic transmission fluid 50. It had some good looking specs and is GL4 rated. You wouldn't need something as thick in your AX15, but it would probably be smooth running.
 

mstrjon32

Thread starter
Messages
1,565
Location
Elkridge, MD
The AX15 is a light-duty transmission and sees mostly light-duty use from me, so nothing too crazy is needed. I've heard (read?) some very good things about MT-90 in the AX15, so I'll keep you posted on how it goes. The AX15 shifts like [censored] with the Coastal fluid in there, so I'm really looking forward to getting it changed out with the Redline. I also find it rather odd that it's OK to use GL-5 in pre-'96 AX5/15 transmissions. Why did they change the material of the synchros?
 
Messages
8,859
Location
Texas
 Originally Posted By: mstrjon32
I also find it rather odd that it's OK to use GL-5 in pre-'96 AX5/15 transmissions. Why did they change the material of the synchros?
That silliness by Aisin is the biggest (only) disappointment of my Jeep ownership experience. I knew the Aisin AW4 automatic they use shifts like sludge but is reliable, so I found a stick Cherokee. Too bad I didn't realize the post-96 AX15 had the synchro issue until later. After 2000, Chrysler started putting a real manual transmission (NV3550) in the Cherokee, so if my AX15 needs an overhaul (synchros already don't like going into first despite loading it up with MT-90) an NV3550 is going in its place.
 

mstrjon32

Thread starter
Messages
1,565
Location
Elkridge, MD
 Originally Posted By: 440Magnum
That silliness by Aisin is the biggest (only) disappointment of my Jeep ownership experience. I knew the Aisin AW4 automatic they use shifts like sludge but is reliable, so I found a stick Cherokee. Too bad I didn't realize the post-96 AX15 had the synchro issue until later. After 2000, Chrysler started putting a real manual transmission (NV3550) in the Cherokee, so if my AX15 needs an overhaul (synchros already don't like going into first despite loading it up with MT-90) an NV3550 is going in its place.
Mine will not go into 1st while I'm moving unless I double clutch the shift. I basically never shift down into 1st while I'm moving, and it's not really needed with the gearing on this thing anyway. The bigger issue is that unless you feather it into 2nd, it grinds and 3rd has become quite notchy and generally unpleasant. Oddly, downshifts into 2nd are always perfect, it's just upshifting from first into 2nd that is difficult. If this transmission were to go I'd probably replace it with the NV3550, as well. I'm hopeful it will be a lot better with the new fluid.
 
Messages
39,806
Location
Pottstown, PA
The factory spec GL-3 synthetic is $14/quart at the dealer. It's the only GL-3 fluid available that I know of. It's not a repackaged OTC product. That said, any MT rated fluid will work. The shifting performance will vary with (mostly) the visc. Even with the factory synth, it's a beeatch to get smoothly in 2nd when cold.
 
Last edited:

mstrjon32

Thread starter
Messages
1,565
Location
Elkridge, MD
Finally had a chance to do a little work on the car today. Drained the Coastal 80W-90 that was in there and filled it with some Rotella T 15W-40 as a flush before I fill it with $50 worth of Redline MT-90. It seemed a little wacky with the 15w-90 at first, but now feels just as good as it did with the Coastal 80w-90 (which is not good at all). I'll drive it until next week with the 15w-40 in there and then fill it with the MT-90. I'll keep posted on how she feels.
 
Messages
768
Location
Portland, Oregon
The AX-15 in my '96 Dakota never shifted all that great even with Redline fluid. The box did have GL-5 in it previously so it's entirely possible there was already damage done to the syncros.
 
Messages
1,513
Location
Peterborough
If thats the standard tranny, they are aparently quite happy on the lighter synchromesh fluids. I think the Mobil fluid is GL5 which is a nono for brass. Try the amsoil synchromesh or GM synchromesh. It should help your situation.
 
Messages
8,859
Location
Texas
 Originally Posted By: ryan2022
If thats the standard tranny, they are aparently quite happy on the lighter synchromesh fluids. I think the Mobil fluid is GL5 which is a nono for brass. Try the amsoil synchromesh or GM synchromesh. It should help your situation.
Careful... there were several standard transmissions in the Cherokee over the years. By far the most common is the AX-15, but its an Aisin (also used by Toyota) transmission that is spec'ed for a 90-wt fluid, not a light ATF-like synchromesh fluid. I've been very tempted to try one of those fluids in my AX-15 because it already shifts like poo on everything else, but its not what is recommended. The last manual transmission in the Cherokee (2000 and 2001 models) and the one also used in 2000-up Wranglers is a Chrysler/NVG 3550, and it is meant for light synchromesh fluids. What I really should do is rip out the AX-15 and put in an NV3550... but since lots of people have that idea they're pretty pricey and rare in junkyards :-/
 

mstrjon32

Thread starter
Messages
1,565
Location
Elkridge, MD
A quick update: I drained the 15w-40 motor oil and filled it with the Redline MT-90. The shifting didn't seem to improve at first but after a thousand miles of driving it does seem quite a bit smoother. The shifts from 1->2 and 2->3 are definitely improved, but 3rd is still notchier than it should be. Either way, I think it was money well spent.
 
Top